The present invention relates to a pelletizing step in a process for recovering metals such as copper, nickel, molybdenum and cobalt from manganese deep sea nodules. For the purpose of this patent specification and claims, complex ores which are found on the deep sea floor of oceans and lakes containing manganese, iron, copper, nickel, molybdenum, cobalt and other metal values are variously referred to as deep sea manganese nodules, manganese nodules or nodules.
Ocean floor deposits are found as nodules, loose-lying at the surface of the soft sea floor sediment, as grains in the sea floor sediments, as crusts on ocean floor hard rock outcrops, as replacement fillings in calcareous debris and animal remains, and in other less important forms. Samples of this ore material can readily be recovered on the ocean floor by drag dredging, a method used by oceanographers for many years, or by deep sea hydraulic dredging, a method that could be used in commercial operation to mine these deposits. Mechanical deep sea nodule harvesters are described in U.S. Pat. Nos. 3,480,326 and 3,504,943.
The character and chemical content of the deep sea nodules may vary widely depending upon the region from which the nodules are obtained. The Mineral Resources of the Sea, John L. Mero, Elsevier Oceanography Series, Elsevier Publishing Company 1965, discusses on pages 127-241 various aspects of manganese nodules. For a detailed chemical analysis of nodules from the Pacific Ocean see pages 449 and 450 in The Encyclopedia of Oceanography, edited by R. W. Fairbridge, Reinhold Publishing Crop., N.Y. 1966, and U.S. Pat. No. 3,169,856. For the purpose of this invention, the complex ores will be considered as containing the following approximate metal content range on a dry basis:
METAL CONTENT ANALYSIS RANGE ______________________________________ Copper 0.8 - 1.8% Nickel 1.0 - 2.0% Cobalt 0.1 - 0.5% Molybdenum 0.03 - 0.1% Manganese 10.0 - 40.0% Iron 4.0 - 25.0% ______________________________________
The remainder of the ore consists of oxygen as oxides, clay minerals (such as montmorillonite) with lesser amounts of quartz, apatite, biotite, sodium and potassium feldspars and water of hydration. Of the many ingredients making up the manganese nodules, copper and nickel are emphasized because, from an economic standpoint, they are the most significant metals in most of the ocean floor ores.
In U.S. Pat. No. 3,788,841, entitled Recovery of Metal Values from Manganese Nodules, the teachings of which are incorportated herein by reference, the desirability of pelletizing manganese nodules prior to pyrometallurgical treatment is disclosed. By pelletizing the nodule feed, prior to pyrometallurgical treatment, many advantages result. For example, pelletization produces a coarse material which is more easily handled during the reduction stage of the process. Pelletization also increases the intimacy of the reductant when added with the nodule material prior to pelletizing. Pelletization also minimizes the presence of fines which can be troublesome during the leaching stages of the process. Furthermore, pelletization of the nodules contributes to the production of tailings which are more easily disposable.
Pelletization per se is well known in the smelting art. Typically, balls or green balls are made by feeding ground ore, additives and moisture independently on to a rotating pan, disc or drum. Agglomerates are also made by mixing the ground ores, additives and moisture, and pressing the mixture into the form of pellets or briquettes. The terms "balls" and "pellets" will be used interchangeably throughout this specification.
A common additive which serves as a bonding agent for the pellets is a sodium bentonite. High sodium bentonite swells upon water addition making the clay useful as a binding agent. Bentonites are composed chiefly of montmorillonites which are clay minerals generally distinguished from other clay minerals by their surface activity and extremely fine particle sizes.
The general formula for montmorillonite is EQU (1/2 Ca, Na,).sub.0.7 (Al,Mg,Fe).sub.4 (Si,Al).sub.8 (OH).sub.8.nH.sub.2 O;
the general formula for high sodium montmorillonite is Na.sub.0.7 (Al,Mg,Fe).sub.4 (Si,Al).sub.8 (OH).sub.4.nH.sub.2 O; and,
the general formula for high calcium montmorillonite is 1/2 Ca.sub.0.7 (Al,Mg,Fe).sub.4 (Si,Al).sub.8 (OH).sub.4.nH.sub.2 O.
bentonites are defined as fine grained clays containing not less than 85% montmorillonite. Two broad divisions of bentonites are sodium bentonite (which expands in water and contains sodium as its predominant exchangeable ion) and calcium bentonite (which has negligible swelling and carries calcium as its principal exchangeable ion).
Of course, it would be an economic advantage to be able to produce strong pellets of manganese nodules without the extra expense of adding a bonding agent such as sodium bentonite.
In short, if a bonding agent such as sodium bentonite is eliminated from the pelletizing procedure, weak pellets result. On the other hand, adding sodium bentonite to the manganese nodule material increases the cost of producing the pellets.